Sizing solar panels: 1600W consumption and battery bank recharge

Tmp4000
Tmp4000 Registered Users Posts: 34 ✭✭
edited June 2017 in Solar Beginners Corner #1
Can you guys verify if my math makes sense ?


  • I am planning on having a system that will consume 1600W non-stop, all day. The peak sun hours will be 6 hours (an average daily insolation of 6 kWh/m^2).
  • The panels that I will use have a 315W output, but the actual NOCT output is 228W.
  • The inverter efficiency is about 92%, miscellaneous 93% 
  • Only have budget for one day of autonomy/downtime

Battery Bank Calculation
======================
Energy needed:  1600W * 24hrs = 38,400Wh  ->  Apply efficiencies --> 38,400Wh/(0.92*0.93) = 44,881Wh

Using a DoD of 70%: and 48V Bank:  44,881Wh/48V = 936Ah  -->936 Ah / 0.7 = 1336 Ah of battery bank needed

So from this 1336Ah battery bank I will draw 936 Ah every day.


Solar Panels Needed (for recharging + equipment usage)
================================================

To Recharge Batteries during the peak 6 hour windows:

Amount of battery energy needed to be replaced:  936 Ah that need to be replaced --> 936 Ah * 48V = 44,928Wh of energy needed

If using 228W panels (actually 315W):  228W*6h =1368Wh per panel  --> 44,928Wh/1365Wh = 33 panels 

So I need 33 panels to recharge my battery bank in 6 hours of full sunlight


However, I would also like to use the solar panels to power the 1600W equipment WHILE also recharging batteries:

So while the Sun shines for 6 hours, my equipment will draw:  1600W*6hrs = 9600Wh of energy

The # of panels needed:  9600Wh/ (228W*6h) = 8 panels

TOTAL Panels the System will need:
=================================================
33 Panels to recharge batteries + 8 panel to power the equipment during sunshine = 41 panels


Am I doing this correctly ? 

What I don't understand is what happens to the solar energy once the battery bank is charged. Will the controller - inverter send solar power straight into the inverter/appliances bypassing the battery bank ? That doesn't make sense...since the Inverter is connected to the battery bank, not to the panels.

What I was thinking is that once the battery bank is charged, can the power from from the solar panels goes straight into the Inverter ? However, obviously the inverter is connected to the batteries. So once the batteries are charged and there is a load on the inverter, the current draw will come from the panels through the battery bank and discharge the battery bank again, correct ?

Is there a way to draw power straight from the panels only once the battery bank is charged ? 


Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    One derating factor that you may have missed... 80% flooded cell battery efficiency or 90%+ AGM battery efficiency.

    There is also one other derating factor that you could plug in--That is the ~95% charge controller efficiency. You used NOCT which is ~72% for that panel--I usually use 77% derating for panels+controller--So you are already a bit more "conservative" that I normally use. However, if your system is in a very hot climate and installed near sea level/humid area--That 72% derating (NOCT) + another 95% derating (controller) may be needed.
    • 33 panels * 1/0.90 AGM battery derating * 1/0.95 MPPT charge controller derating = 39 panels
    And if you do the same 0.95 derating for your panels to run the load, then:
    • 8 panels * 1/0.95 controller derate = 8.4 panels -- Not much different
    Otherwise, your calculations look OK. If your loads are one for 1-2 days, then off for a day or two--That does give you battery bank a chance to fully recharge--There are just not enough hours in a day for a Lead Acid bank to fully recharge from 70% DoD every day. Of course, ideally in sunny weather, you are only running from the battery bank ~18 hours per day and from the solar array the other ~6+ hours.

    I still believe the 6 hours of sun per day for a fixed array is a bit optimistic for most regions of the world except for high deserts. You cold add tracking, or more panels, or split your array with 1/2 the array tiled east and the other 1/2 tilted west (virtual tracking--You would need a somewhat larger array for same output and "more charging hours per day", but (possibly) cheaper than mounting your 1/3rd fewer panels on full two axis array--) or you could go with "real" 2-axis tracking. Just to give you an idea of how much that could affect your output, using PV Watts for KITALE, KENYA, flat array vs a 2-axis tracking:
    http://pvwatts.nrel.gov/pvwatts.php

    Flat array (zero degree tilt)
    MonthSolar Radiation
    ( kWh / m2 / day )
    January5.81
    February6.19
    March5.77
    April5.16
    May5.18
    June4.89
    July4.87
    August5.20
    September5.63
    October5.55
    November5.26
    December5.57
    Annual5.42

    2-axis tracking:
    MonthSolar Radiation
    ( kWh / m2 / day )
    January8.09
    February8.52
    March7.30
    April6.31
    May6.64
    June6.38
    July6.08
    August6.51
    September7.10
    October6.95
    November6.75
    December7.68
    Annual7.03

    The above is based on "real data" for an equatorial system. Could do some more work with a spread sheet of the hourly data, split east/west array, and see how many hours of "useful" sun you can get.

    At this point, solar panel are the "cheap part" of your system (vs battery bank). I would try to error on the "larger array" side. That will give you more flexibility and a "happier" battery bank. Will also give you some extra capacity in case of failures (failed panel, battery cell/bank capacity issues, scientists want "more power").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tmp4000
    Tmp4000 Registered Users Posts: 34 ✭✭
    edited June 2017 #3
    The battery system will probably be Lithium Iron LiFePO4 (i.e. like the one from Iron Edison). We might be able to find more funds for the battery bank type (using LiFePO4 instead of wet traditional batteries) if I can convince the right people. We will be located about 1 degree North of the Equator. The panels will stay flat on a makeshift roof, no tracking.

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited June 2017 #4
    If you can swing LiFePO4 batteries--That would be great. Are you looking at a Battery Management System for the LFPO batteries? Charge controller needs to have zero temperature compensation and programmed for max voltage. You also need min-voltage power disconnect/shutdown (or similar) to prevent ruining your bank.

    Will the battery/power system be in a separate building? Or at least a sealed off room from the rest of the equipment. Battery (and genset) fires are not common, but reducing your exposure to fire/smoke for expensive equipment is not a bad thing.

    Can you gives us a Longitude as well? That can tell us a whole bunch more about the weather altitude/etc.

    If this is a long way off (year or more)--Can you get a weather station + solar energy for logging out there? Or even a USB logger and a solar cell would tell you a lot

    http://www.davisnet.com
    https://www.dataq.com/resources/pdfs/datasheets/el-data-logger.pdf
    https://www.lascarelectronics.com/easylog-data-logger-el-usb-4/

    $80+small solar panel (a "shorted solar panel", i.e., measuring current will give you solar radiation within ~5%-10% accuracy or so).. 1 year battery life, 22 months of logging with 30 minute sample.

    Whole thing the size of your hand (or smaller).

    -Bill

    You could even build a small scale model of your system and install it out there--And log/monitor the results. A nice/small MPPT charge controller and a couple hundred watts of panels--Plus some sort of programmable load on the battery to simulate your proposed profile.

    Morningstar SunSaver 15 Amp MPPT Solar Charge Controller ($245)

    -BB
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tmp4000
    Tmp4000 Registered Users Posts: 34 ✭✭
    BB. said:
    If you can swing LiFePO4 batteries--That would be great. Are you looking at a Battery Management System for the LFPO batteries? Charge controller needs to have zero temperature compensation and programmed for max voltage. You also need min-voltage power disconnect/shutdown (or similar) to prevent ruining your bank.

    Will the battery/power system be in a separate building? Or at least a sealed off room from the rest of the equipment. Battery (and genset) fires are not common, but reducing your exposure to fire/smoke for expensive equipment is not a bad thing.

    Can you gives us a Longitude as well? That can tell us a whole bunch more about the weather altitude/etc.

    If this is a long way off (year or more)--Can you get a weather station + solar energy for logging out there? Or even a USB logger and a solar cell would tell you a lot

    http://www.davisnet.com
    https://www.dataq.com/resources/pdfs/datasheets/el-data-logger.pdf
    https://www.lascarelectronics.com/easylog-data-logger-el-usb-4/

    $80+small solar panel (a "shorted solar panel", i.e., measuring current will give you solar radiation within ~5%-10% accuracy or so).. 1 year battery life, 22 months of logging with 30 minute sample.

    Whole thing the size of your hand (or smaller).

    -Bill

    You could even build a small scale model of your system and install it out there--And log/monitor the results. A nice/small MPPT charge controller and a couple hundred watts of panels--Plus some sort of programmable load on the battery to simulate your proposed profile.

    Morningstar SunSaver 15 Amp MPPT Solar Charge Controller ($245)

    -BB
    We talked to Iron Edison and they said their batteries come with a Battery Management System. I will need to ask if that includes minimum voltage shutdown.

    The location is extremely remote. It will take a few planes and then several hours of freight boat to get to on location. We will have unloading help with shipping docks. We will use several 8' x 20' shipping containers:  one will house our equipment and the other will house the solar system w/battery banks. Looking if maybe we can get a third one. Each containers will be cooled by a 5000 Btu/h air conditioner. The cooling system will have its own solar electrical system and the rest will have its own.

    We are a small group of scientists/engineers that have this project coming up soon. We didn't work with such a large solar electrical system, so we are looking for any secondary assistance/recommendation we can find. 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    You will want your loads to shut down. Not the battery bank (would require manual intervention to recharge). Over and under voltage can immediately ruin LFPO cells.

    Our host, NAWS, (and other companies) can provide design support and various levels of preconfigured systems.

    Also, there are "epanels" that are prewired for various charge controllers and inverters.

    You will also find a number of discussions on sailing and RV forums about LiFePO4 batteries too.

    - Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Be sure you are talking about Lithium batteries when you are conferring with Iron Edison! I did a double take as I didn't know they did Lithium only knew about Nickel-Iron batteries (which have an  even worse charging profile!).
    BB. said:.... You used NOCT which is ~72% for that panel--I usually use 77% derating for panels+controller--So you are already a bit more "conservative" that I normally use. However, if your system is in a very hot climate and installed near sea level/humid area--That 72% derating (NOCT) + another 95% derating (controller) may be needed....
    Bill, I think he is using the companies NOCT values, I've been seeing mostly 75% and less for a couple years.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.